1. Field of the invention
The present invention relates to a microwave oven, more particularly to a power supply circuit for driving a magnetron equipped in a microwave oven, which provides stable power to the magnetron by preventing instability of outputs voltage due to LC resonance between a high voltage condenser (HVC) for driving the magnetron and a secondary winding of a transformer, and by attaining a good insulation between the secondary windings of the transformer in a switching mode power supply employed with a pulse width modulation.
2. Description of the Prior Art
In general, high voltage is required to drive a magnetron equipped in a microwave oven. A conventional power supply producing such a high voltage drives the magnetron by the high voltage induced to a secondary side by switching intermittingly a primary current.
According to the conventional power supply, if a cut-off period of the primary current is varied, the voltage for driving the magnetron is changed, thereby, allowing the output of the magnetron to be controlled appropriately.
Such a technique is disclosed in Japanese Patent Laid-open publication No. Sho 53-27143 (MAGNETRON DRIVING POWER SUPPLY) and Japanese Patent Laid-Open No. Hei 2-135690 (MICROWAVE OVEN).
A magnetron driving power supply according to the Japanese Patent Laid-open publication No. Sho 53-27143 is composed of a low frequency oscillator enabling change of the duty cycle of an output signal, a high frequency oscillator for controlling a signal transmission or signal oscillation by using the output signal from the low frequency oscillator, and a switching circuit for switching a direct current applied to a primary side of an output transformer of which a secondary side is connected to the magnetron, thereby obtaining a stable output on the basis of the input voltage.
In a microwave oven disclosed in Japanese Patent Laid-open publication No. Hei 2-135690, an output voltage is not varied with the frequency change of commercial power by intermitting a primary current of a transformer in accordance with each phase of the different frequencies of the commercial power to constantly ensure an output level of a magnetron.
With the construction as described above, however, a HVC must be used when generating high voltage by using a high voltage transformer (HVT). The magnetron will be damaged by the instability of output voltage of the HVT due to, LC resonance between the HVC and a secondary winding of the HVT connected to a magnetron. Further, there is a problem that a microwave oven is likely to be damaged by an unstable power supply voltage, applied to the magnetron, causing the destruction of insulation between winding coils of the transformer by the high voltage at a secondary side of the transformer.